Mining stocks lead shares lower

The Australian sharemarket was half a per cent lower on Monday afternoon, with the big miners leading the way down following a negative lead from the US.

At 1.48pm AEDT, the benchmark S&P/ASX 200 index was down 19.6 points, or 0.5 per cent, at 4253.5.

Cameron Securities client adviser Adrian Leppinus said to AAP the Australian sharemarket had gradually weakened throughout the morning.

"The resources sector has come off after commodities were generally a bit weaker on Friday," Mr Leppinus said.

"The general feeling is the market has had a reasonable start following a mixed reporting season."

Investors would look towards offshore leads while still keeping an eye on the situation in Europe, he said.

At noon the nation's big miners were all weaker, with Rio Tinto suffering the heaviest losses at 1.45pm AEDT.

Rio was 1.5 per cent lower while BHP Billiton was 1.3 per cent lower.

The big banks were mixed, with ANZ up 0.7 per cent, but CBA, NAB and Westpac all about half a per cent lower.

Optus's parent company, Singapore Telecommunications, gained 0.4 per cent after saying it would restructure the business into three separate units from April 1, 2012.

Optus chief executive Paul O'Sullivan would lead one of those units, the Group Consumer business, the telco said.

In other news, Qantas Airways pilots are challenging a ban that prevents them from taking industrial action.

Fair Work Australia (FWA) imposed the ban last year after a long-running spell of industrial action by Qantas pilots, engineers and ground staff that led to the airline's entire fleet being grounded in October.